This invention relates to a process for preparing pigment compositions having improved dispersibility in plastics and other macromolecular materials by conditioning organic pigments with non-solvents containing certain surfactants containing N-oxide groups.
Organic pigments in the form initially obtained after chemical synthesis, often referred to as crude pigments, are generally unsuitable for use as pigments and must be subjected to one or more additional finishing steps that modify particle size, particle shape, and/or crystal structure in such a way that provides good pigmentary quality. See, for example, K. Merkle and H. Schafer, "Surface Treatment of Organic Pigments" in Pigment Handbook, Vol. III (New York: John Wiley & Sons, Inc., 1973), page 157; R. B. McKay, "The Development of Organic Pigments with Particular Reference to Physical Form and Consequent Behavior in Use" in Rev. Prog. Coloration, 10, 25-32 (1979); and R. B. McKay, "Control of the application performance of classical organic pigments" in JOCCA, 89-93 (1989).
The most commonly used conditioning methods involve dissolving or suspending the crude pigment in strong mineral acids, followed by precipitation, and/or milling the crude pigment. Conditioning with a strong acid involves treating the crude pigment with aqueous mineral acid such as sulfuric acid in a process known as "acid pasting" (in which an acidic solution containing protonated pigment is formed) or "acid swelling" (in which a suspension of protonated pigment is formed). After the acid treatment is completed, the pigment is precipitated by adding the strongly acidic solution to a liquid in which the pigments are completely or almost completely insoluble, such as water or methanol or other lower aliphatic alcohols, as well as mixtures thereof.
Further treatment of conditioned organic pigments is sometimes desirable or necessary, particularly when the pigments are to be dispersed in plastics. Surface treatment is a type of finishing in which certain auxiliaries, such as rosin or other resins, are applied to pigments to influence their surface structure and thus their physical and coloristic properties. E.g., W. Herbst and K. Hunger, Industrial Organic Pigments (New York: VCH Publishers, Inc., 1993), pages 205-207. For example, treatment of organic pigments with emulsions of certain dispersing agents such as sulfonated dicarboxylic acids in non-aqueous volatile oils such as naphtha can improve the dispersibility of the pigments in non-aqueous vehicles used for inks, paints, and varnishes. E.g., U.S. Pat. No. 2,268,144. The elimination of such additional steps would be advantageous if desirable pigmentary properties could be maintained.
U.S. Pat. No. 5,662,739 describes a method for improving the dispersibility of quinacridone and dioxazine pigments by using certain fatty acid taurides. The fatty acid taurides, however, are amides rather than ammonium or amine compounds such as used in the present invention.
European Patent Application 758,004 describes a method for improving the dispersibility for a specific pigment, Pigment Yellow 12, by carrying out the preparative coupling reaction in the presence of certain cationic amine and amine oxide surfactants. The European application, however, does not mention other types of pigments and does not suggest that pigments could be conditioned in the presence of such surfactants.
An object of the present invention was reducing or eliminating the use of strong acids and eliminating further surface treatment steps while at the same time providing organic pigments that can be easily dispersed in plastics. These and other objects have been achieved by conditioning organic pigments with non-solvents containing specific types of surfactants containing N-oxide groups.